1. Field of the Invention
The present invention relates to systems and methods for horizontal directional drilling and more particularly to the use of a self-controlled, rotary steerable system for use in horizontal direction drilling.
2. Description of the Related Art
Horizontal directional drilling is the application of drilling techniques to steer a drill along a prescribed pathway beneath an obstacle such as a river or beach. This pathway is then enlarged and improved such that a pipeline or conduit can be installed beneath the obstacle. The drill path takes a line below the surface to avoid disturbance of the banks or beach and thereby greatly reduces environmental impact. Commonly, the drill path may be 30 or 40 feet beneath the surface.
Since the surface of the banks or beach are not disturbed, detrimental effects on water quality, vegetation, or wildlife are minimized. Additionally, by drilling beneath the surface of the beach, the risk of erosion is reduced or eliminated. Typically, a drilling rig is set up behind the beach or sand dunes. From there, a pilot hole is drilled at an angle to the surface. The hole continues horizontally below the surface of the beach (typically 30–40 feet below the surface) and exits at a remote submerged location after crossing beneath the beach. Once the pilot drill assembly exits the bore at a submerged location, it is commonly lifted to a barge where a reamer is attached to enlarge the hole. The reamer is drawn back through the hole and the hole is enlarged to roughly 1½ times the diameter of the product conduit. The product conduit is then pulled through the hole from the offshore end. Drilling fluid is pumped through the hole during the drilling and reaming operation. Sufficient volumes of fluid must be pumped to maintain sufficient velocities to adequately remove the drilled cuttings from the hole. The fluid volumes are on the order of 400–600 gpm during the drilling of the pilot hole and may be even higher during the reaming process. Commonly, the drilling fluid contains clay additives to provide sufficient gel strength and viscosity to aid in transporting the drilled cuttings from the borehole. The drilling fluid with cuttings typically exits the hole at the subsea end and the drilling cuttings and clay particles are allowed to settle on the seafloor. The large flow volumes result in a substantial amount of particulate matter being deposited. The cuttings and gel material are normally benign materials. However, environmentally sensitive structures, such as coral reefs, may be damaged by the deposition of large amounts of such material. The result is that the horizontal reach of the borehole is being pushed farther and farther offshore. In some areas, lengths greater than 10,000 ft are required.
Horizontal directional drilling is commonly accomplished by use of a special drilling rig employing a non-rotating drill pipe with a fluid powered cutting tool at its downhole end. Direction is achieved by use of a small angular section in the body of the cutting tool, and by controlling the application of thrust on the drill string. Downhole drilling motors may be used to rotate the bit. In addition, wireline steering tools have been used to determine the path of the long reach borehole, as described in U.S. Pat. No. 4,399,877 to Jackson, et al. Horizontal lengths of 4000–6000 ft are not uncommon using such techniques. Use of such a wireline tool prevents the use of a rotary drilling system.
The limits of the prior art techniques are caused by the friction induced drag of the drill pipe as it lays against the wall of the pilot hole. In addition, the relatively flexible drill pipe tends to buckle as the thrust load is increased, exacerbating the problem. The use of larger diameter, and therefore stiffer, drill pipe may alleviate the buckling problem but aggravates the frictional drag by increasing the weight of the drill pipe. U.S. Pat. No. 6,443,244 to Collins describes the use of buoyant sections of drill string to partially reduce the frictional drag. The resultant sections are substantially larger in diameter and while partially reducing the weight, they drastically increase the surface area in contact with the cuttings on the bottom of the hole and the drag of such a non-rotating system is still to great to prevent very long reach drilling.
The methods and apparatus of the present invention overcome the foregoing disadvantages of the prior art by providing a rotary steerable system and methods for drilling a very long reach borehole while reducing the impact on environmentally sensitive areas.